Dental plaque develops as a complex, multispecies biofilm on the tooth surface and is a direct precursor of periodontal diseases, one of the most common bacterial infections of humans. The general principles of plaque biofilm development and transition from a commensal to a pathogenic entity are established. Pioneer colonizers such as oral streptococci first attach and accumulated on the tooth surface. Through a variety of adhesive, signaling and physiologic interactions, these early colonizers then foster the colonization of later arrivals, including potential pathogens such as P. gingivalis. Our ongoing studies address the stage of biofilm development involving initial attachment of planktonic cells of P. gingivalis to a substratum of S. gordonii, transition of P. gingivalis to a biofilm lifestyle, and development of an architecturally distinct heterotypic biofilm. Adhesion of P. gingivalis to S. gordonii is mediated by the long and short fimbriae of P. gingivalis that interact with streptococcal GAPDH and Ssp surface proteins. Within SspB, the amino acids in an adhesive domain spanning residues 1167-1193, designated BAR, dictate binding activity and specificity. The interaction of SspB with the short fimbrial subunit protein, Mfa, induces a distinct transcriptional profile within P. gingivalis that provides the impetus for subsequent accumulation of a mixed species biofilm. One such regulated gene, ptpA, encodes a tyrosine phosphatase that participates in a control mechanism to optimize the size of the heterotypic biofilm community. Longer term adaptation to the community lifestyle causes P. gingivalis to downregulate expression of Mfa. The objectives of this application are to characterize the SspB-Mfa interacting interface; to define the role of the P. gingivalis tyrosine phosphatase PtpA in regulatory networks that control heterotypic biofilm development with S. gordonii; and to characterize the transcriptional regulation of mfa expression. We shall utilize global approaches such as transcriptomics and proteomics in combination with targeted approaches such as site specific mutagenesis, protein-protein and protein-DNA binding assays, to define and dissect the functions of individual biofilm effector molecules and to reveal the hierarchical network structure that controls heterotypic biofilm development. Public Health Significance: Of relevance to public health is the potential to translate this knowledge into the development of toothpastes or mouthrinses that contain inhibitors of the process of P. gingivalis build-up on the teeth. These would reduce the ability of P. gingivalis to persist in the mouth and consequently prevent or reduce the severity of gum disease caused by this organism.